Loop heat pipe for equipment cooling

ABSTRACT

The apparatus is a loop heat pipe with the heat input above the heat output, so that the loop heat pipe is used to cool a transformer or equipment enclosure above ground with the condenser located and the heat being dissipated in the ground below the location at which the heat is being generated. One embodiment uses a coiled pipe for a condenser and buries it in the ground encased in concrete. Another embodiment of the invention is used to cool a diesel electric locomotive by placing the condenser below the engine and attaching multiple cooling fins to it.

BACKGROUND OF THE INVENTION

[0001] This invention deals generally with cooling electrical equipment, and more specifically with a loop heat pipe arranged for cooling heat generating devices for which it is impractical to install cooling systems at the same height as or above the heat source.

[0002] At the same time that heat generation from electrical devices is increasing because of significant increases in the electrical power being used, environmental concerns are making it more difficult to cool many electrical devices in the traditional manner.

[0003] For example, the typical means for cooling a building which holds telephone switching or radio transmitting equipment is to mount a heat exchanger or air conditioner condenser in a window-like opening in the building and to use a high volume, high speed fan to expel the heat into the atmosphere around the building. That is all very well when the building is located on an isolated site remote from any people, but it is becoming increasingly necessary to locate such buildings in or near population centers. When people or other buildings are nearby, the noise and heat produced by such cooling systems are at least an annoyance, and may also be illegal.

[0004] For high power electrical transformers, the type commonly seen in electrical substations, the problem is similar. Such transformers have previously been cooled by natural convection or by small, relatively quiet fans, but the trend is to increase the power rating of such transformers, and thus the heat load is becoming more difficult to handle. In fact, to avoid unsightly surface installations the trend is to install such transformers underground, sometimes in or below building basements, and that creates a significant heat disposal problem.

[0005] A somewhat different situation arises in the cooling of diesel electric locomotives. In that case, short term heavy loads, such as those caused by moving up a steep grade, make it necessary to store heat until the heavy load is reduced and the cooling system can dispose of the stored heat. It would be very desirable to dissipate the extra heat load rather than store it, but there is simply no available space in the typical locomotive to expose a heat dissipating radiator to external air.

SUMMARY OF THE INVENTION

[0006] Each of the cooling problems discussed above can be solved by the use of a loop heat pipe constructed with the heat dissipating condenser at a level below that of the heat generating component. For the cooling of electrical equipment buildings and transformers, loop heat pipes transfer the generated heat directly from the heat generating device to the soil below the site, and for increased heat transfer to the soil the condenser of the loop heat pipe is embedded in a concrete body. Such a concrete structure can actually be part of the foundation of the building in which the heat generating device is located. The advantage of loop heat pipes is their ability to transfer heat against gravity, and to deliver heat to a large extended surface, such as an underground structure, without the use of any external power.

[0007] Loop heat pipes are well known in the art of heat transfer. Superficially they appear to be a simple closed loop of pipe with a larger diameter section at the heat input point. They are used for transferring heat because they enable the heat transfer system to operate without the external power required for operating mechanical pumps to circulate the heat transfer fluid. In a loop heat pipe the heat input into the system is the only power needed to circulate the heat transfer fluid, so that loop heat pipes have been particularly valuable where electrical power for mechanical pumps is unavailable or difficult to access. For example, loop heat pipes have been used in spacecraft where the power consumption of a conventional pump motor is too high.

[0008] Loop heat pipes operate somewhat similarly to a conventional still. Liquid entering the pump is vaporized by heat applied to it at an evaporator section, and the vapor then moves out of the pump to a cooler location in the system. At the cooler location, which is the condenser, the vapor condenses and the liquid condensate is either returned to the pump or to a liquid reservoir which feeds the pump. It is the differences in vapor pressure throughout the system which move both the vapor and the liquid.

[0009] There are, however, significant differences between a loop heat pipe and a simple still, the major one being that the loop heat pipe includes a capillary pump and a pressure differential to return the condensed liquid to the heat input region to be evaporated again. A porous wick separates the liquid inlet of the capillary pump from its vapor outlet and, unlike a still, in which heat is applied directly to the liquid, the input heat to drive the capillary pump, which is usually supplied from a device which is being cooled, is applied to this separating wick. The heat applied to the wick causes liquid, which has been moved into the wick by capillary action, to be vaporized and to move out of the wick and along pipes in the system to the condenser where the vapor is condensed. Such capillary pumps are often mounted adjacent and parallel to other capillary pumps on a cooling plate upon which are mounted heat producing components which are being cooled.

[0010] A typical capillary pump is built as a cylindrical configuration so that heat can be applied to the outside of the cylinder. In such a configuration, the liquid enters into the center of the cylinder from one end, and the capillary wick forms the outer cylindrical boundary of the liquid chamber, which is closed off at the liquid chamber end remote from the liquid entry pipe by a simple plug. An annular vapor chamber surrounds the capillary wick, with the outer casing of the pump forming the outermost boundary of the annular vapor chamber, and the outer casing extends longitudinally beyond the capillary wick where it essentially becomes a hollow pipe which can be connected to other pipes to transport the vapor to the condenser. Thus the typical capillary pump essentially has liquid entering one end of a heated cylinder and has vapor leaving the other end.

[0011] In order for the external heat to be more effectively transferred to the wick through the annular vapor chamber, the vapor chamber is usually constructed as a group of longitudinal channels within the inner wall of the casing or the outer surface of the wick, with the side walls of the channels actually contacting the adjacent capillary wick or casing. This structure with longitudinal grooves permits the external heat to be conducted directly to the wick through the sidewalls of the grooves, and to evaporate the liquid which is in the wick, yet it provides space within the grooves for the vapor to escape from the wick.

[0012] To aid movement of the condensed liquid from the condenser back to the evaporator of the capillary pump, the prior art loop heat pipes typically are constructed with the condenser at the same height or higher than the capillary pump. This orientation then uses gravity to help the movement of the liquid back to the capillary wick of the capillary pump. Although the structure of such loop heat pipes is conventional, the subtle limitation of such a structure is that the source of heat is almost always located at a lower level than the cooling condenser. However, such devices are not taking full advantage of the capability of a loop heat pipe. With a properly designed loop heat pipe, the condenser of the loop heat pipe can be placed well below the evaporator, and differences in internal pressure in the loop heat pipe move the liquid back to the capillary pump evaporator wick. Experimental operation has demonstrated that a condenser ten to thirty feet below the evaporator is quite practical.

[0013] The present invention is therefore a loop heat pipe with the condenser located below the capillary pump evaporator, and a heat sink designed to take full advantage of the heat transfer of the loop heat pipe. It has two important applications.

[0014] The first is for cooling electrical equipment enclosures and transformers. In such applications the capillary pump is located at the heat generating source, the electrical equipment or the transformers, and the condenser is located lower than the capillary pump, underground where the heat can be dissipated into the soil. However, the heat conductivity of soil is rather poor, and it limits the effective distance of heat transfer to only a foot or so from the condenser pipe of the loop heat pipe. To counteract this limitation, the condenser is constructed as a helix or some other layout for which the length of the total structure is significantly shorter than the length of the pipe from which it is constructed. Such a structure greatly increases the surface area of the pipe and the heat transfer area for the condenser, and the increased heat transfer area facilitates heat transfer into the soil even though soil has a relatively poor heat conductivity.

[0015] To further improve the heat transfer, the helix is embedded in a solid concrete body poured around the condenser in the ground. Although concrete is not a good heat conductor relative to metals, it is significantly better than the typical soil, so that the solid concrete body acts as both a heat storage body and a means of increasing the soil surface area to which the heat is transferred.

[0016] The second application for the invention is cooling of diesel electric locomotives. The advantage in this application is the ability to locate an air cooled radiator below the engine, in an otherwise unused location, which is, however, exposed to outside air. Thus, the capillary pump evaporator is located at the engine component to be cooled, and the loop condenser is located under the engine, between the wheels. In that location, a conventional heat exchanger, such as an assembly of fins, is attached to the condenser section of the loop heat pipe, and the heat exchanger is exposed to the air flowing below the locomotive. A layout of the condenser pipe as multiple parallel paths or a single serpentine layout can be used to increase the heat transfer area available to the outside air.

[0017] The invention thereby furnishes improved and environmentally friendly cooling for two applications for which cooling has previously been difficult.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a simplified schematic drawing of the invention installed in a manner to cool an electrical equipment enclosure.

[0019]FIG. 2 is a simplified schematic drawing of an alternate embodiment of the invention installed to cool components in a locomotive.

DETAILED DESCRIPTION OF THE INVENTION

[0020]FIG. 1 is a simplified schematic drawing of loop heat pipe 10 of the invention installed in a manner to cool an electrical equipment enclosure 12 within a building 14.

[0021] The evaporator of loop heat pipe 10 is included within capillary pump 16 in the conventional construction of a loop heat pipe, and is connected to condenser section 18, which is a simple length of pipe, by pipes 20 and 22, either of which could be the vapor pipe or the liquid return, depending upon the orientation of capillary pump 16. For the present invention, condenser section 18 is located below capillary pump 16.

[0022] Capillary pump 16 is built conventionally as a cylindrical configuration so that heat can be applied to the outside of the cylinder. In such a configuration, the liquid enters into the center of the cylinder from, for example, pipe 20 at one end, and a capillary wick forms an outer cylindrical boundary of a liquid chamber, with the liquid chamber closed off at the end remote from liquid entry pipe 20. An annular vapor chamber surrounds the capillary wick and the liquid chamber, with the outer casing of the pump forming the outermost boundary of the annular vapor chamber. The outer casing extends longitudinally beyond the capillary wick where it essentially becomes a hollow pipe which is connected to vapor pipe 22 which transports the vapor generated in capillary pump 16 to condenser 18.

[0023] In the preferred embodiment of the present invention, loop heat pipe 10 is installed with a substantial portion of it, at least entire condenser section 18, below ground level 24 and below the frost line 25. This permits the soil to act as a heat sink to cool condenser section 18 which condenses the vapor within loop heat pipe 10. To take full advantage of the relatively constant 55 degree Fahrenheit temperature of the soil below frost line 25, condenser section 18 is installed below ground level 24 by a distance A which should be a minimum of 24 inches.

[0024] For loop heat pipe 10, condenser 18 is constructed as a helix to increase the surface area of the pipe relative to the space occupied by the entire structure and to simplify installation. However, the invention also functions satisfactorily when the condenser is constructed as a straight length of pipe or some other geometry such as a serpentine configuration.

[0025] Since soil is a relatively poor conductor of heat, the operation of the invention can also be improved by encasing condenser section 18 in concrete which is a better heat conductor than most soils. Concrete body 26 is therefore shown in FIG. 1 completely encasing condenser section 18. With such a structure, concrete body 26 not only withdraws heat from condenser section 18, but acts as a heat storage medium, and since it provides a much larger surface area than the area of the pipes of condenser section 18, it aids in conducting heat to the surrounding soil. It also can be very advantageous to install condenser section 18 within part of a structure of a building, such as foundation 27.

[0026] Typical dimensions for the structure of loop heat pipe 10 can be up to a total length of 20 to 30 feet, with a typical tubing size of ¼ to 1.0 inch internal diameter, and copper, stainless steel, or low carbon steel used as the material of the loop heat pipe. Condenser 18 has 6 to 12 turns with the helix 12 to 18 inches in outer diameter, and 2 to 3-½ feet long depending upon the power being transferred. Capillary pump 16 has an O.D. of 1 to 2 inches and a length of 4 to 8 inches. Its casing is constructed of the same material as the rest of the loop heat pipe, and its internal capillary wick is constructed of sintered metal powder. In the preferred embodiment, the top of condenser section 18 is installed below frost line 25. Concrete body 26 is constructed of conventional concrete and is 18 to 24 inches in diameter and 2 to 4 feet long.

[0027] Such a structure can dissipate 10 to 100 watts into the soil with a typical temperature difference of 5 to 20 degrees centigrade between the heat input to capillary pump 16 and the soil temperature.

[0028]FIG. 2 is a simplified schematic drawing of an alternate embodiment of the invention with loop heat pipe 30 installed to cool component 32 of locomotive 34. Loop heat pipe 30 has capillary pump 36 located in proximity to heat generating component 32, and has condenser section 38 located outside locomotive 34, below the undercarriage of locomotive 34, and lower than capillary pump 36.

[0029] Condenser section 38 is constructed in a serpentine configuration with assemblies of multiple cooling fins 40 attached to the pipe segments of condenser section 38. Each pipe segment is oriented across the width of locomotive 34 so that each fin 40 is oriented parallel to the direction of the air stream flowing past the engine. Thus, loop heat pipe 30 furnishes additional cooling for the locomotive and utilizes space which is not otherwise utilized.

[0030] The present invention thereby furnishes a means for cooling equipment which is extremely simple to install and operate. The invention requires no electrical or mechanical interconnection to operate a pump or fan and produces no noise. The invention actually operates with no moving parts, which minimizes maintenance requirements, and the underground embodiment does not dissipate heat into the surrounding air.

[0031] It is to be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the spirit and scope of the invention as defined in the following claims. 

What is claimed as new and for which Letters Patent of the United States are desired to be secured is:
 1. A loop heat pipe for cooling a heat generating device comprising: a capillary pump located in proximity to and heated by a heat generating device, with the capillary pump including a liquid input and a vapor output; a condenser section which is constructed of pipe, located below the capillary pump, and installed underground; a vapor pipe interconnecting the condenser section to the vapor output of the capillary pump; a liquid return pipe interconnecting the condenser section to the liquid input of the capillary pump; and a heat dissipating means in contact with and cooling the condenser section.
 2. The loop heat pipe of claim 1 wherein the condenser section is formed as a helix.
 3. The loop heat pipe of claim 1 wherein the condenser section is formed into a serpentine configuration.
 4. The loop heat pipe of claim 1 wherein the heat dissipating means is soil surrounding the condenser section.
 5. The loop heat pipe of claim 1 wherein the heat dissipating means is a concrete body formed around the condenser section.
 6. The loop heat pipe of claim 1 wherein the heat dissipating means is a concrete body formed around the condenser section, and the concrete body is located underground surrounded by soil.
 7. The loop heat pipe of claim 1 wherein the heat dissipating means is a concrete body formed around the condenser section, and the concrete body is part of a building.
 8. The loop heat pipe of claim 1 wherein the condenser section is installed underground and below the frost line.
 9. The loop heat pipe of claim 1 wherein the heat generating device is electrical equipment.
 10. The loop heat pipe of claim 1 wherein the heat generating device is electrical equipment installed within a building.
 11. A loop heat pipe for cooling a heat generating device comprising: a capillary pump located in proximity to and heated by a heat generating device, with the capillary pump including a liquid input and a vapor output; a condenser section which is constructed of pipe, located below the capillary pump, and installed aboard a locomotive; a vapor pipe interconnecting the condenser section to the vapor output of the capillary pump; a liquid return pipe interconnecting the condenser section to the liquid input of the capillary pump; and a heat dissipating means in contact with and cooling the condenser section.
 12. The loop heat pipe of claim 1 wherein the loop heat pipe is installed aboard a locomotive, the capillary pump is installed within the locomotive, and the condenser section is installed below the undercarriage of the locomotive.
 13. The loop heat pipe of claim 1 wherein the loop heat pipe is installed aboard a locomotive, the condenser section is installed below the undercarriage of the locomotive, and the heat dissipating means is a group of cooling fins attached to the condenser section. 